Arrangement for position-selective speed measurement using the doppler principle

ABSTRACT

When measuring speed over ground by means of microwave Doppler radar, signal noise, switching spikes and nearby reflectors reduce the measurement accuracy. In order to avoid these disadvantages, a reference signal which recurs with the period T 0  and a target signal which follows the reference signal after a time interval T s  are emitted via an antenna. That part of the reference signal which has already been received again during transmission of the target signal is subtracted from the target signal which is received later. The difference signal obtained in this way is a measurement signal from which interference spikes, signal noise and nearby reflectors have been largely removed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an arrangement forposition-selective speed measurement using the Doppler principle.

2. Description of the Related Art

Cost-effective microwave Doppler sensors are used to measure the speedover ground of land vehicles and vehicles on rails, and for movementdetection in alarm systems. The sensitivity of these sensors isessentially limited by the noise behavior of the microwave generator.

In the case of Doppler sensors which operate using the continuous-wave(CW) mode, position-selective Doppler evaluation is also not possible.

In the Pulse-Doppler method, short microwave pulses are emitted whichrecur at a pulse repetition frequency PRF. The range selection inPulse-Doppler sensors is governed by the time interval between thesignals received from reflection objects at different ranges.

In the case of the Pulse-Doppler short-range sensor, fine positionresolution is desirable, and a short pulse duration and thus a hightransmission bandwidth are required for this purpose. The meantransmitted power and thus the sensitivity of the receiving stage aswell are, however, reduced at the same time. This can be counter-actedby a greater transmitted pulse power or a higher pulse repetitionfrequency PRF, although this limits the maximum unambiguous range area,or by pulse integration, although this limits the maximum speed whichcan be measured unambiguously. Switching feedback effects in the pulsemode make it necessary to install expensive, direction-selectivecomponents such as isolators or separate transmitting and receivingantennas, for example, in the known pulsed sensors.

FIG. 1 shows the block diagram of a Pulse-Doppler sensor as is knownfrom the "Taschenbuch der Hochfrequenztechnik" Radio-frequencytechnology pocketbook!, Chapter S1, Springer Press, 5th Edition, 1992.The microwave signal produced by the radio-frequency generator HFG--amicrowave oscillator--is switched over with the aid of a microwaveswitch S having an amplitude-contrast ratio k: ##EQU1## where: A₁=amplitude during the switching state α

A₂ =amplitude during the switching state β

α=switching state during which the microwave switch MWS is switched on

β=switching state during which the microwave switch MWS is switched off

between two switching states α and β, and is emitted via atransmitting/receiving antenna A. The transmitted field strength E_(S)and the received field strength E_(R) are super-imposed between theradio-frequency generator HFG and the reflection object to form astanding wave. One cost-effective option for obtaining Doppler signalsis homodyne detection by using a Schottky diode to detect the standingwave. The voltage u_(Det) (t) at the demodulator DM, also called thedetector, turns out, because of the square-law diode characteristic inthe CW mode, to be:

    u.sub.Det (t)≡(E.sub.S +E.sub.R).sup.2 ≈E.sub.S.sup.2 =E.sub.R.sup.2 +2E.sub.S E.sub.R cos(φ(t))≈E.sub.S.sup.2 +2E.sub.S E.sub.R cos(φ(t))

where:

E_(S) =transmitted field strength

E_(R) =received field strength

2E_(S) E_(R) cos(φ(t))=wanted signal (Doppler signal)

E_(R) <<E_(S)

Φ(t)=phase shift between the transmitted field strength E_(S) and thereceived field strength E_(R).

Because of the noise from the radio-frequency generator HFG, the termE_(S) ² which corresponds to the rectified element of the oscillatorsignal is not constant. The reception sensitivity is determined by theratio of the fluctuation width of the term E_(S) ² to the amplitude ofthe Doppler signal. In contrast to the CW mode, a Doppler signal ispresent at the demodulator DM in the pulse mode (in the ideal case theamplitude-contrast ratio k is much greater than 1) only in the switchingstate α, α designating the pass mode, and can be sampled at thedemodulator DM, the measurement of the Doppler signal being repeated ata repetition frequency PRF by repeatedly switching over between thestates α and β. A position-selective, continuous-time Doppler signal canbe reconstructed from the sample pulses by interpolation of all thesamples from the Doppler signal, for example with the aid of a sampleand hold element AH. One disadvantage of this method is that the samplesare adversely affected by switching spikes and oscillator noise and,because of the finite amplitude-contrast ratio k<∞, also contain asignal element from undesired range areas. In the past, thesedisadvantages have been only partially overcome using microwave switcheswith high switching contrast, direction-selective elements such asisolators and circulators, or separate transmitting/receiving antennas.These measures considerably increase the costs and circuit complexity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an arrangement forposition-selective speed measurement, in which interference in the formof signal noise, switching spikes and signals from undesirable rangeareas or nearby reflectors is reduced to a major extent, and themeasurement accuracy can thus be increased.

This and other objects and advantages of the invention are achieved byan arrangement for position-selective speed measurement using theDoppler principle

in which a means is provided for producing a pulse sequence which has areference pulse and a target pulse,

in which a periodic signal, which is produced by a radio-frequencygenerator, is applied to an antenna using a means for switching as afunction of the pulse sequence,

in which the signals, which are demodulated by a demodulator which isconnected to the radio-frequency generator and to the means forswitching, are applied to a means for signal sampling and to a means forsubtraction which forms the difference between the signals which aredemodulated during the occurrence of the reference pulse and of thetarget pulse, and

in which the target pulse is applied after the time interval to thecontrol input of the means for switching, after which the periodicsignal which is emitted during the occurrence of the reference pulse canbe received again.

A simple design is possible for producing the switching signal (pulsesequence).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to thefigures.

FIG. 1 shows a schematic view of an arrangement for position-selectivespeed measurement as is known from the publication Taschenbuch derHochfrequenztechnik Radio-frequency technology pocketbook!, Chapter S1,Springer Press, 5th Edition 1992.

FIG. 2 shows a schematic view of arrangement according to the inventionfor position-selective speed measurement.

FIG. 3 shows a series of graphs with a Minkowski diagram, whichcorresponds to the speed measurement arrangement, and associated typicalsignal waveforms.

FIG. 4 shows a circuit diagram of an alternative arrangement forproducing a pulse sequence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pulse generator PG which is illustrated in FIG. 2 produces a firstpulse which recurs with the period T₀ and is called the reference pulseRP, in the following text. This reference pulse signal passes via twolines L and LV, of different lengths, to the control input SE of theswitching element S. That part of the reference pulse signal which takesthe longer path, via the delay element LV passes, delayed by the timeinterval T_(s) with respect to the signal element of the reference pulseRP which is routed via the shorter line L, to the control input SE ofthe switching element S. A reference pulse RP, which recurs with theperiod T₀ and a pulse, called the target pulse ZP, which likewise recurswith the period T₀ and is shifted by the pulse interval T_(s) withrespect to the reference pulse RP are thus applied to the control inputSE of the switching element S.

When one of the two pulses RP or ZP is applied to the control input SEof the switching element S, then a microwave signal MS is switchedthrough to the antenna A for the pulse duration T_(p), which microwavesignal MS corresponds, for this period, to the radio-frequency signal S1produced by the radio-frequency generator HFG. The microwave signal MSwhich is emitted from the antenna A is the pulse-modulated signal fromthe radio-frequency signal S1 and from the reference pulse RP, or thepulse-modulated signal from the radio-frequency signal S1 and from thetarget pulse ZP, or a residual signal which does not decay to zerobecause of the finite switching contrast of the switching element S.Since the switching element S is switched on, the received signal (anecho) also passes through the demodulator DM at the same time so that aDoppler signal S2 is produced there, which can be sampled. The pulseinterval T_(s) is set such that the echo of the radio-frequency signalS1 which is emitted while the reference pulse RP is present and isreflected from the range s=c·T_(s) /2 arrives at the same time as thetarget pulse ZP occurs, and passes to the demodulator DM. While thetarget pulse ZP is present at the control input SE of the switchingelement S, a radio-frequency signal S1 which is modulated with thetarget pulse ZP is emitted once again. The second demodulated signal S2(t₂) which is present at the output of the demodulator DM during theoccurrence of the target pulse ZP at the time t₂, contains in additionto all the interference signals the information about the reflector orbackscattering elements, while the first demodulated signal S2 (t₀),which is present at the time t₀ of occurrence of the reference pulse RPat the demodulator output, contains only the interference elements. Thecomparison (the subtraction) of the two demodulated signals S2 (t₀) andS2 (t₂) produces a signal S3 from which the interference has beenremoved and which contains only the information about the reflector orreflecting object. In this case, both the generator noise as well as inthe instances of switching feedback effects and of the nearby reflectorsand reflections from undesired range areas are eliminated because of thefinite switching contrast k<∞. This results in a considerable increasein sensitivity and in the intended increase in position selectivity.

The measurement process is repeated with a period T₀. In this case, T₀is selected to be sufficiently long that the measurements (target,reference) run independently of one another, that is to say there are nolonger any received signals from the preceding measurement process ineach new measurement.

The reflections of the transmitted signals can be illustrated in theposition/time domain using the Minkowski diagram shown in FIG. 3. Timeis plotted on the abscissa, and position on the ordinate. The ordinateis broken down into three areas, the close-range area FN, the targetarea FZ and the long-range area FK, the close-range area FN and thetarget area FZ being important for operation. At the time to at whichthe reference pulse RP is applied to the control input SE of theswitching element S, the switching element S, also called a microwaveswitch, is switched on for the time for which the reference pulse RPoccurs, which corresponds to the switching state α. The continuousradio-frequency signal S1 produced by the radio-frequency generator HFGis applied to the antenna A, modulated with the reference pulse RP, andis emitted for the time during which the switching element S is switchedon. At the time t₂ at which the target pulse ZP is applied to thecontrol input SE of the switching element S, the radio-frequency signalS1 is modulated with the target pulse ZP and is applied to the antenna Afor the pulse duration T_(p) and is emitted. During this pulse durationT_(p), the signal which was emitted at the time to and was subsequentlyreflected is at the same time received at the antenna A.

The signal elements S2 (t₀) and S2 (t₂) which contain the signal S2,present at the demodulator output, at the time t=t₀, t=t₂ are shown inthe following table:

    ______________________________________                                        Signal/time          t = t0 t = t2                                            ______________________________________                                        Short-range reflection                                                                             X      X                                                 Switching peaks      X      X                                                 Oscillator noise     X      X                                                 CW element (because k < ∞)                                                                   X      X                                                 Target reflection           X                                                 ______________________________________                                    

The difference between the two signal elements is a signal S3 from whichall interference has been removed.

Sampling of the signal which is supplied from the demodulator, withsimultaneous subtraction, is carried out by a special sample and holdelement AH with a subtraction element DIF.

The demodulated signal S2, comprising the pulse sequence S2 (t₀) and S2(t₂) is connected via a line LL1 to a sampling switch AS and is passedto the memory capacitance CS when the signal S2 (t₂) is present at thepoint AE. The length of the line LL2, which is short-circuited at theend, is selected such that the inverse signal element S2 (t₀) is alsopresent at the same time at the point AE at the time at which the signalelement S2 (t₂) is present at the point AE, so that, overall, thedifference is passed through to the memory capacitance CS. The voltageinversion is carried out by a radio-frequency short-circuit by acapacitance CK at the end of the line LL2. Passing the signal at thepoint AE through to the memory capacitance CS is initiated by a controlpulse SE2, which can be derived from the pulse generator PG.

One alternative arrangement for producing the pulse sequence may appearas is shown in FIG. 4. A positive pulse P produced by the pulsegenerator PG is passed through the diode Di and is split between the twolines LV and L. That element of the pulse P which has passed into theline LV is reflected at the open-circuit end of line LV with +1 andpasses, delayed by the pulse interval T_(s) with respect to the otherelement, to the control input SE of the switching element S. Thereference pulse RP thus corresponds to that element of the pulse P whichis passed only through the line L while, in contrast, the target pulseZP corresponds to that element of the pulse P which is passed throughboth the line LV and the line L.

The switching element S can be replaced by an element for amplitudemodulation. The pulse sequence is not limited only to pulses. Any signalwaveforms can be used which are suitable for modulation of theradio-frequency signal S1.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventor to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of his contribution to the art.

I claim:
 1. An arrangement for position-selective speed measurement using the Doppler principle, comprising:a means for producing a pulse sequence which has a reference pulse and a target pulse, a radio-frequency generator, a means for switching said periodic signal as a function of the pulse sequence, a demodulator which is connected to the radio-frequency generator and to the means for switching, a means for signal sampling a means for subtraction for forming a difference between the demodulated signals which are demodulated during occurrence of the reference pulse and of the target pulse, and the target pulse after a time interval to the control input of the means for switching, after which the periodic signal which is emitted during occurrence of the reference pulse can be received again.
 2. An arrangement as claimed in claim 1, whereinthe means for producing the pulse sequence includes a pulse generator for producing the reference pulse, and a delay element for producing the target pulse.
 3. An arrangement as claimed in claim 2, whereinthe delay element includes a delay line which is connected in parallel with a line, the delay line is longer than the line by a length which corresponds to the time interval.
 4. An arrangement as claimed in claim 2, whereinthe delay element includes a delay line which has an open circuit at one of its ends and is connected to a diode and to a line at its other end, and the diode is connected to the pulse generator.
 5. An arrangement as claimed in claim 1, wherein,the means for subtraction includes a memory in which the demodulated signal is stored until a next demodulated signal is available.
 6. An arrangement as claimed in claim 5, whereinthe memory of the means for subtraction has a short-circuited conductor at whose end inversion of the demodulated signal takes place, a length of the short-circuited conductor corresponds to the time interval.
 7. An arrangement as claimed in claim 1, whereinthe means for switching is a means for amplitude modulation. 